DEFINITION OF RELIABILITY

Reliability
is the probability of a device performing its purpose adequately
for the period of time intended under the operating condition encountered.

A failure-rate
curve of the type shown in Fig. 1 has been called " bath tub " curve.

The failure
early period OA is sometimes called the infant-mortality or the
burn-in period. Failure occurrence can be discerned by burn in or
aging of the components.

The failure rate
will decrease to a lower value ( Point C ) which remain fairly constant
and define the beginning of normal operation period. During this
period, failure takes place at random. The important objective of
the manufacturer is to minimize this failure rate as low as possible.

In
general, if X represents a constant probability of failure per
hour, the probability of an item surviving for H hours is (1-X
)H. For small X one can approximate:

R(t)=exp
(-HX ) where R(t) is the probability of survival after H hours.
and 1/X =MTBF ( Mean Time Between Failure )